In an image taking optical system constituting an image pickup apparatus, a zoom lens is required to have a short overall lens length (distance from a first lens surface to an image plane), to be compact, to have high resolution, and to have a high zoom ratio. In addition, the zoom lens is required to have a wide angle of view and a short focal length at a wide angle end so that a wide image taking range can be obtained even if an image taking distance is short.
In general, in order to reduce the size of the zoom lens while achieving a high zoom ratio, the number of lenses may be reduced while the refractive power of each lens unit making up the zoom lens is increased. However, if the refractive power of each lens surface is increased, it becomes difficult to correct various aberrations.
If, for compactly accommodating each lens unit when an image is not taken, a structure that allows each lens unit to be collapsed is used, an assembly error, such as falling of the lenses and the lens units, tends to increase. Here, if the sensitivities of the lenses and the lens units are high, optical performance is reduced. Therefore, it is desirable that sensitivities with respect to decentering of the lenses and the lens units in the zoom lens be small.
Hitherto, a four-unit zoom lens including, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power is known.
U.S. Pat. No. 7,190,529 discusses a zoom lens having a zoom ratio of approximately 10. In this zoom lens, a change in lateral magnification of a second lens unit during zooming is small. Therefore, if, in order to further increase the zoom ratio, the refractive power of the second lens unit is increased or the burden of varying magnification allotted to the other lens units is increased, it is difficult to restrict variations in aberrations during zooming.
U.S. Pat. No. 7,206,137 discusses a zoom lens having a zoom ratio of approximately 15 to 20. However, measures for solving problems regarding various aberrations, such as curvature of field and off-axis color flare when widening the angle of view, are not satisfactorily discussed.
Japanese Patent Laid-Open No. 2007-003554 discusses a zoom lens having a zoom ratio of approximately 18. This zoom lens has a high zoom ratio and a wide angle of view. However, the amount by which a first lens unit is moved out is relatively large. Therefore, in order to reduce the overall lens length when the lens unit is collapsed, a lens barrel needs to be a multi-step lens barrel, as a result of which the structure of the lens barrel becomes complicated.
US Patent Application Publication No. 2006/0146417 discusses a zoom lens that corrects image blur by vibrating the entire third lens unit perpendicularly to an optical axis.
If the aforementioned four-unit zoom lens is reduced in size, the effective diameter of the first lens unit is strongly influenced by incidence height of off-axis light with respect to a lens surface in a region extending from a wide angle end to an intermediate zooming area. Therefore, in order to reduce the size of the zoom lens, it becomes important to suitably set the refractive power of each lens unit, the amount of movement during zooming, and a movement locus including the intermediate zooming area.